This invention relates in general to digital radiography, and in particular to the imaging of a long human body part, such as the spine or legs, using a storage phosphor-based computed radiography system.
When a long segment of the human body is imaged using the conventional screen-film technique, special cassettes and films of extended length are used, such as 30xc3x9790 cm and 35xc3x97105 cm. As medical institutions are migrating from analog screen-film systems to digital modalities, such as computed radiography (CR), these types of exams impose a significant challenge. This is because the size of digital detector is limited. For example, the largest CR storage phosphor cassette from several major CR vendors is limited to 35xc3x9743 cm, which can only image a partial of the body part at a time. To address this problem, a method has been proposed that staggers several storage phosphor cassettes together in a specially made cassette holder (U.S. Pat. No. 5,986,279, European Patent App. EP0866342A1, EP0919856A1, and EP0919858A1). All the cassettes are exposed in a single x-ray exposure. Then image processing is applied to stitch all the partial images together. The advantage is that the method is compatible with the current CR readers. However, a pattern of reference markers needs to be imaged simultaneously with the patient in order to achieve precise geometric registration of the partial images. The shadow of the reference markers may obscure diagnostically important information in the stitched image. Also because of the overlapping of the cassettes, the metallic cassette frames introduce wide shadow artifacts in the resultant image that are sometimes objectionable. Moreover, the cassette holder is quite heavy and is typically mounted in a fixed position, which limits the users from moving it up and down for exact patient positioning. The cassette holder is bulky and does not conform to ISO/ANSI standards, which means that it can not be placed in the bucky grid holder that is designed for the current screen-film systems. U.S. patent application Ser. No.: 09/742,509 filed Dec. 20, 2000, discloses a method that is based on an extended length cassette with two 35xc3x9743 cm phosphor screens built inside. The two phosphor screens are slightly overlapped in the center of the cassette (FIGS. 1-3). The overall cassette size is about 35xc3x9785 cm, which nearly doubles the current largest cassette size and allows a fairly long segment of the human body to be imaged at a single exposure. The information recorded in either phosphor screen bears part of the desired final image.
During the readout process, one end of the cassette is placed in the CR reader and the first phosphor screen is scanned and stored, the cassette is then removed from the reader and inverted to allow the second phosphor screen to be read in the same manner as the first. The two images can then be processed into a composite full image if so desired. The length of the cassette can be designed to be shorter or longer in order to follow the ISO/ANSI standard, such as 36xe2x80x3 and 51xe2x80x3 inch long. The maximum cassette length is approximately twice the maximum allowable scan length of the CR reader.
Special digital image processing is required to construct a composite full image from the front and back images that are obtained from the two individual phosphor screens. The two phosphor screens are packed and partially overlapped inside the single cassette and are therefore not coplanar. This causes the image of the body part to be magnified differently for different locations in the cassette, and a demagnification operation is required as part of the process of registering the front and back images. In addition, the two phosphor screens will not be perfectly aligned inside the cassette, and there are translation and rotational displacements introduced by the CR reader during the image readout process. As a result, the placement of the pixels from the front and back images will not be perfectly aligned, and the images will require rotation and translation compensation. The aforementioned image registration processing can be accomplished by de-warping the front and back images to a set of reference markers (with known position) that are imaged in conjunction with the body part. However, it is desirable that the images be acquired without the use of any reference markers to preclude the possibility of obscuration of the important diagnostic regions of the image. It is therefore desirable to develop an image processing algorithm that can automatically (1) conduct image demagnification, (2) correct the translation and rotational displacements between the front and back images, and (3) make use of the information in the front and back images to form a composite full image that has high geometric fidelity without relying on any reference markers.
According to the present invention, there is provided a solution to the problems discussed above.
According to a feature of the present invention there is provided a method of forming a composite image from first and second digital images formed by recording first and second contiguous segments of a larger radiographic image in first and second overlapping storage phosphor members, exposed to a source of X-rays wherein the image content in the overlapped region is the same in both images and the end edge of said first member is present both on said first image and as a shadow edge in said second image, said method comprising: correcting for geometric distortion in said first and second digital images; determining any rotational displacement and any vertical displacement between said first and second images by matching said first member end edge in said first image to its shadow in said second image; correcting for image orientation based on any said rotational displacement; determining any horizontal displacement between said first and second images by correlating said image content in said overlapped region of said first and second images; and stitching said first and second images together along said first member end edge based on any said horizontal and vertical displacements.
The invention has the following advantages.
1. Enables the generation of a full composite image from two partial images that is free from artifacts by completely eliminating the use of references.
2. Preserves a high degree of geometric accuracy in the stitched image.